雙相機攝影法推估對流雲發展高度與對流特徵–以2017年夏季北台灣午後對流個案為例

Abstract

熱帶地區的對流發展是由三種對流雲所組成，其中介於淺對流與深對流之間的濃積雲，除了在深淺對流轉換中扮演關鍵角色。近年來已出現許多相關研究，不過對於濃積雲的發展高度仍然缺乏明確客觀的界定。 雙相機攝影測量是一種利用兩台相機來測量物體於三度空間中位置的技術，本研究利用該觀測技術，在桃園濱海架設兩台自動化相機朝北台灣山區方向進行觀測，並利用地面山脈與夜空恆星來取得影像的方位角與仰角，計算出北台灣2017年6月到9月發生午後對流時的對流雲頂高度。最後選出五天在時間與發展高度觀測最完整與清楚的個案進行觀測結果分析，並分析每個高度區間的垂直上升速度、垂直上升加速度與雲頂寬度，找出濃積雲與深對流的差異。 分析結果顯示，對流雲在往上發展時，垂直上升加速度會隨高度上升減速程度越來越大，直到約8-9公里突然有較大的加速度，然後再隨著高度上升而下降；雲頂寬度則是隨高度上升越來越寬，直到約8-9公里突然變窄，再隨著高度上升而越來越寬；垂直上升速度則是在8-9公里以下常呈現穩定上升的趨勢，此外並沒有發現明顯一致的特徵。分析各高度區間對流雲數量的結果顯示，對流雲在大約7-12公里上下會出現數量低值，與垂直上升加速度、雲頂寬度出現特徵的高度相當接近但不一致。

Tropical convective cloud has been observed to contain three cloud modes. Cumulus congestus clouds between cumulus and deep convection plays an important role in the transition from shallow to deep convection. Many related studies have improved the understanding of cumulus congestus, but there is still lack of clear and objective definition of its cloud top height. Stereo-photogrammetry is a technique that uses two cameras to measure the position of an object in three-dimensional coordinates. This research uses this technique to set up two automated cameras at Taoyuan coastal toward mountian, and use the mountains and stars in the image to obtain the azimuth and elevation to calculate cloud top height of afternoon convection in northern Taiwan from June to September 2017. Five of the most complete and clearest cases are selected to analysis. And analyzes the vertical velocity, vertical acceleration and cloud top width of each altitude level to find the difference between cumulus congestus and deep convection. The results indicate that when convective cloud develops upwards, the vertical acceleration will decelerate with height and suddenly increase until it reaching 8-9 km, and then keep decelerate with height; the width of cloud top is exactly the opposite of the acceleration change.; the vertical velocity tends to increase steadily with height below 8-9 km. The results of analyzing the amount of convective clouds in each height level show that the convective cloud will have a low value around 7-12 km, which is close but not consistent with the result of vertical acceleration and cloud top width.